Detectors used, or attempted to be used, for the detection of gamma and X-rays, in particular in CT, SPECT and PET systems, include inter alia direct-converting detectors based on semi-conductive materials, such as CdTe, CdZnTe, CdZnTeSe, CdTeSe, CdMnTe, InP, TIBr2, HgI2. Monocrystals or polycrystals in these materials have numerous crystal defects, for example lattice defects, impurity atoms and/or deliberately introduced dopants. These crystal defects generate inter alia fixed, electronic states with bonding energy lying between the energies of the valence and conduction band of the semiconductor. Here, a differentiation is made between flat impurities, which are energetically close, that is less than 30 meV, to the valence or conduction band, and deep impurities, with an energetic spacing to the bands of greater than 30 meV. Flat impurities are already fully ionized at room temperature. This means that their occupation probability does not change as the temperature increases, in particular during the operation of a CT device. However, at room temperature, the deep impurities are only partially ionized so that their occupation probability is heavily temperature-dependent. This applies to semiconductor detectors both with and without additional charge-carrier generation by additional irradiation.
The changing occupation probability of the deep impurities in the event of temperature fluctuations causes a change in the electric field in the interior of the semiconductor and hence also results in a change in the electric power. This also causes a change in the pulse shapes of the pulses triggered by the X-rays and, in the case of firmly fixed electronic thresholds for the pulse-height discriminators of the detector, there is also a change in the counting rates in the case of a constant X-ray flux. This results in the following problems: on the one hand, due to the photocurrent generated, the inserted X-ray flux results in the heating of the semiconductor and a drift in the counting rate. The associated energy loss P of the detector is determined by the high voltage U applied and the photocurrent I using the equation P=U*I. On the other hand, calibration tables, which were generated under specific temperature and flow conditions, lose their validity as soon as the temperature of the semiconductor changes. Both effects result in unacceptable artifacts in the imaging.
Hitherto, it is known how to regulate, i.e. stabilize, the temperature in the semiconductor material by means of control elements, for example Peltier elements, which are attached under the unit comprising semiconductor and evaluation electronics, the ASIC (=Application Specific Integrated Circuit). However, this has the following drawbacks: the temperature regulation is inert. Temperature fluctuations, caused by a rapidly changing X-ray flux cannot, therefore, be compensated. However, in a CT system and also in other imaging methods, typically above all rapid changes to the X-ray flux occur. Moreover, with the known temperature regulation, the heat or the temperature change is generated, not in the interior of the semiconductor, but outside the semiconductor, that is on its surface. This inevitably results in a non-uniform temperature profile in the semiconductor since, for example, the surface is exposed to a cooling air flow. Therefore, on the insertion of an X-ray flux, the temperatures in the semiconductor can change even if the average temperature remains constant. This also causes a change in the electric field in the semiconductor and hence a counting rate drift.